There are various kinds of flat pens, such as markers, felt-tip pens, fiber-tip pens, fluorescent markers, whiteboard pens, and others that require assembly using various parts. The assembly procedure of a flat pen is mainly in the form of an assembly line, wherein the pens are assembled piece by piece in different, separate assembly processes. The assembly procedure of a flat pen mainly includes a wraparound-core mounting process, an ink-filling process, a pen-tip mounting process, a pen-cap mounting process, a pen-cap pressing process, and the like. The pen-cap pressing process is a manual operation. The operator wears hardboard on his/her hand and then presses the pen-cap via the hardboard in order to fasten the pen cap. Efficiency can be very low. During the operating procedure, it is easy for the operator to injure his/her hand by the repetitive force of the hardboard. Further, in the assembly procedure in the form of an assembly line, at least one operator is required for each process, so the conventional process is very labor intensive. Furthermore, when one process has been completed, it is necessary to transfer the semi-assembled products to the worktable for the next process. Because the semi-assembled products are often transferred manually, not only is the assembly efficiency very low, but it is easy for the semi-assembled products to be damaged during transferring. The result is waste and low efficiency in assembling pens.